1. Filed of the Invention
The present invention relates to devices to reduce the radiation of electro-magnetic fields from enclosures containing electrical or electronic equipment.
2. Background of the Invention
Metal enclosures are often used to contain electronic circuitry. These enclosures interact with the electro-magnetic fields emitted from the circuitry. In the presence of a high frequency component of the electro-magnetic field, enclosures can sustain resonant fields at multiple frequencies. These fields are either absorbed by lossy (resistive) materials contained within the enclosure or are radiated through gaps in the enclosure to the external environment. Such fields contain a magnetic (H) and an electric (E) component.
Radiation to the external environment compromises Electro-Magnetic Compatibility (EMC) performance and can impair the operation of other electronic circuitry. Electro-Magnetic Interference (EMI) is a problem throughout the electronics industry. EMI is subject to regulation by various authorities because stray fields from electrical and electronic equipment can disrupt other electronic equipment such as avionics and pacemakers. Undesirable radio frequency (RF) energy is a pervasive problem for EMI. Where practical, RF emissions are minimized by circuit design and track layout.
EMI solutions usually entail the containment of the RF energy within an enclosure using shielding and gaskets. Additionally, ferrites are often used to dissipate undesirable RF energy present on signal wires and power cables. The ferrites are physically located close to the target fields. The ferrites requires a direct physical or electrical (galvanic) connection to the electronics.
The invention described herein provides a degree of absorption for magnetic fields and for electric fields within an enclosure which does not rely on shielding, gaskets, or the use of ferrites as a purely absorbing element.
Accordingly, the invention provides apparatus for reducing electromagnetic field radiation within an electrically conducting enclosure comprising an electrically conducting element having two extremities and in which the electromagnetic field radiation induces a current and an electrical resistance connected between the two extremities of the electrically conducting element so as to absorb the electromagnetic field radiation. The element acts to receive the electromagnetic field energy which is then converted to heat energy by the electrical resistance. This has the advantage over the prior art that the energy is dissipated by conversion to heat, rather than contained as is the case when screening and gaskets are used.
In a first embodiment, the element is a substantially linear conducting element which absorbs electric field radiation and the electrical resistance has a direct electrical connection to a first extremity of the electrically conducting element. This embodiment is directed at the absorption of electric field energy.
In a second embodiment, the element is a substantially circular conducting element which absorbs magnetic field radiation, each end of the electrical resistance having a direct connection to a respective extremity of the substantially circular conducting element. This embodiment is directed at the absorption of magnetic field energy.
Preferably, the substantially circular conducting element comprises multiple turns. increased for a given magnetic flux density.
In a preferred embodiment, the element further comprises a high permittivity material located within the loop formed by the element and the resistance. This has the advantage of increasing the magnetic flux within the loop as compared to the magnetic flux density outside the loop and hence the energy absorbed is increased.
In a particularly preferred embodiment, the element is formed on a printed circuit board. Fabrication of the element can then be done by any of the well known techniques used for fabrication of printed circuit boards at no additional cost.
In the present invention, the target fields are physically separate from the circuitry and the device requires no direct physical or electrical (galvanic) connection to the electronics.